I dunno, based on a look or two (heck, I got nothing better to do this morning), I think there must be some kind of a latch under the bottom bracket. Once that is unlatched, the downtube and chainstay move downward until the headtube and seat tube meet up. There may also be some disconnection of the top tube and seat stays.

Got it! The eBay seller was nice enough to tell me how to do it. You squeeze the two bars in the back (behind the rear reflector) -- squeeze them toward each other and the bike basically collapses by itself. Sweet!